Like many I was was blown away by the N9 UI, espacially in the way multitasking is presented.
Harmattan has clearly a significant edge over all competition in this respect (some competition may even never catch up), but I still think that we should start thinking of improvements for the next iteration (granted there will be one). Here are some suggestions, I suppose people may have already formulated similar ideas:
* Currently Harmattan supports a 3x3 or a 2x2 grid in the open application view, with controls of the apps disabled (you can only see the UI but not control it - please correct me if I'm wrong). I propose to add a 2x1 grid (only portrait) with controls *enabled*, which will effectively enable to have 2 applications running side by side. I believe a lot of applications would benefit from this layout (although I can't put my finger on any specific example) and it would be a real differentiator vs. all other platforms. Furthermore this feature would really benefit tablets (with bigger grids) where it is a shame to have a single app covering the whole screen as is currently the case. Access to this 2x1 grid would be through pinch-to-zoom.
* Introduce "open application folders" the same way "application folders" will probably be implemented. An "open application folder" is a group of open application arranged in a certain grid layout (the same way applications are organized in folders). Once you are in an "open application folder" you only see the thumbnails of the apps within the group, or scroll through the open apps within the group. Open application folders are created exactly the same way application folders are, except we operate on and open applications thumbnails instead of application icons. I am not sure about the best way to go from one "open application group" to another: a swipe or maybe a zoom-out.
